1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus having a recording head for jetting liquid droplets.
2. Description of the Related Art
There is known a liquid-jet recording type inkjet recording apparatus, which is an example of an image forming apparatus such as a printer, a fax machine, a copier, a plotter, and a multifunction peripheral having these functions. Such an inkjet recording apparatus uses recording heads for jetting ink droplets. Specifically, ink droplets are jetted from the recording heads onto a conveyed sheet (the sheet is not limited to paper; the sheet may be an OHP film, or any other sheet on which ink droplets and other types of liquid may adhere; the sheet may also be referred to as a recording medium, a recording sheet, etc.) in order to form images (recording, printing, etc., may be used as synonyms). There is a serial type image forming apparatus for forming images by jetting liquid droplets while the recording head moves in the main scanning direction, and a line type image forming apparatus for forming images by using a line type head as the recording head to jet liquid droplets while the recording head is not moving.
In the present application, an “image forming apparatus” means an apparatus for jetting liquid droplets onto a medium made of paper, threads, fiber, a silk cloth, leather, metal, plastic, glass, wood, ceramics, etc. Furthermore, “image forming” does not only mean to form images having meanings such as characters and figures onto a medium, but also to form images without any meanings such as a pattern (to simply jet liquid droplets onto a medium). “Ink” does not only mean what is typically referred to as ink, but also means any liquid that can be used for forming images, such as recording liquid, fixing process liquid, and liquid. Examples are DNA samples, resist, and pattern materials.
In such an image forming apparatus including a device that forms images by jetting liquid droplets, the gap between the recording head and the sheet has a significant impact on image quality. Therefore, the height position of the carriage carrying the recording head is typically adjustable (the height can be changed) with respect to the platen member supporting the sheet. Otherwise, the height position of the platen member (or a conveying belt) is adjustable with respect to the carriage. Accordingly, the gap can be adjusted.
However, cockling (corrugation) may occur in a sheet that has become swollen as a result of absorbing the ink printed on the sheet. The width of the gap between the recording head and the sheet may change depending on the thickness of the sheet, and may also change depending on the extent of cockling. The sheet with cockling may rise from the platen member and be rubbed against the recording head. Printing failures may occur due to the change in the gap between the recording head and the sheet. Accordingly, in order to flatten out the sheet, plural fixed ribs with predetermined intervals are integrally formed on the platen member in the sheet width direction (main scanning direction), so that the sheet can be prevented from rising.
Conventionally, patent document 1 discloses a sheet supporting means for supporting a sheet facing a recording head. Specifically, movable ribs are provided under a sheet-type member for pressing down a recording sheet, and the movable ribs are moved after the sheet passes through a conveying roller. The purpose of this configuration is to prevent the image quality from degrading due to differences in the height at the trailing edge of the sheet after the sheet has passed through the nip portion of the conveying roller.
Patent Document 1: Japanese Laid-Open Patent Application No. 2006-218806
In patent document 2, movable ribs are provided under a sheet-type member for pressing down a recording sheet, similar to patent document 1. The positions of the movable ribs are moved in accordance with the sheet size. The purpose of this configuration is to prevent the image quality from degrading, by controlling cockling that occurs due to differences in sheet sizes.
Patent Document 2: Japanese Laid-Open Patent Application No. 2006-218807
Patent document 3 discloses a configuration of providing a supporting part that moves on a platen in the sheet conveying direction, so that the sheet edges are constantly supported. The purpose of this configuration is to maintain the distance between the sheet and the recording head at a constant distance, and to perform frameless recording at high speed.
Patent Document 3: Japanese Laid-Open Patent Application No. 2007-176093
Patent document 4 discloses a configuration of providing a movable gap adjusting member for changing the gap between the sheet and the recording head. The movable gap adjusting member is disposed across and opposite to the recording head and the sheet conveying path. The purpose of this configuration is to adjust the gap without degrading the image quality, by maintaining the distance between the recording head and the sheet at a constant distance, at the upstream side and the downstream side of the sheet conveying direction.
Patent Document 4: Japanese Laid-Open Patent Application No. H11-348373
However, cockling cannot be prevented simply by moving the recording head and the platen member relatively with each other to adjust the gap. One approach is to provide movable ribs on the platen member. However, the gap between the recording head and the sheet needs to be maintained with high precision in order to maintain the positional accuracy of the liquid droplets landing on the sheet. Thus, the precision of the gap between the recording head and the sheet may decline due to the movement of the ribs supporting the sheet.